Very high data rate soliton regenerator

Details Very high data rate soliton regenerator Very high data rate soliton regenerator

1998-01-05

2001-05-29

Chan, Jason (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06239893

ABSTRACT:

The present invention relates to a very high data rate soliton regenerator, and to a method of regenerating solitons at a very high data rate.
BRIEF DESCRIPTION OF THE DRAWINGS
The transmission of soliton pulses or solitons of hyperbolic secant envelope, in the portion of an optical fiber having abnormal dispersion is a known phenomenon. The transmission of so-called “black” solitons, constituted by pulse gaps in a continuous signal, in the portion of an optical fiber having normal dispersion is also known; in this case the solitons are of a wavelength such that they propagate with negative chromatic dispersion. Both with “white” solitons and with “black” solitons, the non-linearity in the corresponding portion of the fiber used to compensate dispersion of the optical signal. Soliton transmission is modelled in known manner using the non-linear Schrodinger equation.
Various effects limit the transmission of such pulses, such as the jitter imparted by interaction between solitons and the noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, published in Optical Letters, vol. 11, No. 10, pages 665-667. This effect, which is known as the “Gordon-Haus” effect, puts a theoretical limit on the quality or on the rate at which soliton transmission can take place.
Because of the deformation imparted to solitons by transmission, and in particular because of the jitter imparted by the Gordon-Haus effect, considerable efforts are required to ensure that a signal encoded by solitons is conveyed successfully by a transmission system. One of the satisfactory solutions, and perhaps the only solution, for ensuring successful transmission over virtually infinite distances consists in ensuring synchronous modulation of the solitons. This requires both that the solitons be modulated and, in order to synchronize the modulator, that the soliton data frequency be recovered. Those two functions, modulation and clock recovery, must operate at the soliton data frequency, which means that all-optical solutions are particularly useful, i.e. solutions that are controlled optically. In particular, in order to provide alloptical modulation of the soliton signal, it is necessary to produce or to recover an optical sinewave at the corresponding frequency.
Various systems have also been proposed to produce an optical sinewave at the bit frequency. Conventional electro-optical means, such as a directly modulated laser diode, or using external modulation by means of a MachZehnder interferometer, require high frequency electronic components to be used, thereby increasing the cost of devices, and limiting the maximum frequency to about 40 Gbit/s.
For microwave transmission systems, and for the purpose of reducing the phase noise of semiconductor lasers, an article by U. Gliese et al., published in IEEE Photonic Technology Letters, Vol. 4, No. 8, proposes using an optical phase-lock loop (OPLL) to generate microwave signals in the range 3 GHz-18 GHz. The beat signal between two laser sources, one of which is servo-controlled, is compared with the reference microwave oscillator signal. The phase difference signal that results therefrom is used for controlling the power supply current to the servo-controlled laser source. This ensures that the beat signal is locked onto the reference oscillator, in spite of laser phase noise.
An article by H. Bülow, published in IEEE Electronics Letters, Vol. 31, No. 22, describes a principle of optoelectronic synchronization for an optical demultiplexer. A signal demultiplexed at a sub-harmonic frequency is extracted from a multiplexed microwave signal by using a non-linear loop mirror (NOLM) as a switch at the rate of the sub-harmonic frequency. The article proposes using optoelectronic conversion and an electronic phase-lock loop to acquire and track the demultiplexed signal. One of the objectives of that article is to avoid using a fast phase detector such as that described in the article by Gliese et al.
An article by K. L. Hall et al., published in IEEE Photonics Technology Letters, Vol. 7, No. 8, describes an electro-optical phase-lock loop using a non-linear loop mirror as an all-optical comparator of bit phase. A reference source is presented at the input to the NOLM. A voltage-controlled laser source is coupled in the NOLM. Power measured at the outlet from the NOLM is compared with a reference voltage to control the voltage of the controlled source.
OBJECTS AND SUMMARY OF THE INVENTION
The present invention proposes an original and simple solution to the problem of regenerating solitons by synchronous modulation. It makes it possible to use electronic components of low frequency only even with transmissions having a potential of several hundreds of Gbit/s. In addition, the components of the invention are suitable for being submerged and can therefore be used without difficulty in transoceanic transmission systems.
More precisely, the invention proposes a soliton signal regenerator comprising a non-linear optical loop mirror receiving the soliton signal and modulating it with a control signal, and a device for generating the control by beats between two optical light sources in which the frequency of at least one of the two sources is variable.
It is possible to provide means for servo-controlling the variable frequency of at least one of the two sources as a function of the mean power of the modulated soliton signal.
It is also possible to provide means for servo-controlling the variable frequency of at least one of the two sources as a function of the difference between the mean powers of the modulated soliton signals as transmitted and as reflected by the non-linear optical loop mirror.
Finally, it is possible to servo-control the variable frequency of at least one of the two sources by an optical phase-lock loop for locking the beat signal from the two light sources on the soliton signal.
It is also possible to add to all those systems a source at a frequency that is small compared with the data frequency of the soliton signal, the signal from the source being included in the control signal, and synchronous detection means.
For this purpose, the regenerator preferably comprises a mixer for mixing the signal from the source with the signal corresponding to the mean power of the modulated soliton signal or to the mean power difference, and a filter for performing lowpass filtering on the mixed signal.
Advantageously, both light sources are laser sources, with the frequency of at least one of the two sources being servo-controlled by controlling its reference temperature or power supply current.
The frequency of the control signal may be equal to half the bit frequency of the soliton signal. It is also possible to provide for the non-linear loop mirror to have a three-inlet inlet coupler, and for the frequency of the control signal to be equal to the bit frequency of the soliton signal.
The non-linear loop mirror may have a medium that is highly non-linear, such as a chalcogenide fiber or a fiber whose core is doped with germanium.
In an embodiment, the device for generating the control signal supplies two control signals in phase opposition, which are coupled into the mirror at positions that are symmetrical relative to the inlet coupler of the mirror.
The invention also provides a method of regenerating a high rate soliton signal by synchronous modulation in a non-linear loop mirror, by generating the control signal by beating two light sources, the frequency of at least one of the two sources being variable.
The variable frequency of at least one of the two sources is servo-controlled as a function of the mean power of the modulated soliton signal, or as a function of the difference between the mean powers of the modulated soliton signals respectively transmitted and reflected by the non-linear loop mirror, or indeed by an optical phase-lock loop for locking the beat signal from the two light sources onto the soliton signal.
It is possible for a signal of low frequency compared with the data frequency of the s

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